I've never felt like he's insecure at all. At least not in the sense that he's ever expressed feelings of jealousy or seemed bothered about her sexual past. I wouldn't say that he's a confident guy either... just extremely oblivious about the stuff.
I'm calling it now, he's just touched by the idea that his girlfriend was buying him a Christmas present.
It's just a guess but, while Meguro-san doesn't mind being childishly stubborn, MC might be crying because he was caught by Meguro-san when he was being childishly stubborn.
Human is developed with ear, mouth and brain. Open your mouth to ask, spread out your ears to listen and think with your brain. Do not jump into conclusion and misunderstanding shit.
If he's crying bcs he saw her with that other guy I promise I'll kill somebody. Why jump into conclusions rather than just asking. How fucking difficult is that???
But if he's crying bcs he imagines she's buying a present for him then I'm cool with that I guess